Radio Continuum of Galaxies with H

Radio Continuum of Galaxies with H ₂ O-Megamaser-Disks

Dissertation zur

Erlangung des Doktorgrades (Dr. rer. nat.) der

Mathematisch-Naturwissenschaftlichen Fakultät der

Rheinischen Friedrich-Wilhelms-Universität Bonn vorgelegt von

Fateme Kamali

aus

Firoozabad (Iran)

Bonn 2019

Angefertigt mit Genehmigung der Mathematisch-Naturwissenschaftlichen Fakultät der Rheini- schen Friedrich-Wilhelms-Universität Bonn

1. Gutachter: Prof. Dr. Karl M. Menten 2. Gutachter: Prof. Dr. Pavel Kroupa Tag der Promotion: 12.07.2019

Erscheinungsjahr: 2019

To my parents, for their love and support.

Contents

1 Introduction 5

1.1 What are H₂O -megamaser galaxies? . . . 5

1.2 The importance of H₂O -megamaser galaxies . . . 7

1.2.1 Measuring a fundamental cosmological parameter: the Hubble constant 7 1.2.2 Investigating jet-disk connection . . . 9

1.3 Active galactic nuclei, a brief history . . . 10

1.4 What defines an active galaxy? . . . 11

1.5 Active galactic nuclei classification. . . 11

1.5.1 Type 1 and Type 2 AGNs . . . 11

1.5.2 Low excitation and high excitation AGNs . . . 13

1.5.3 Radio quiet and radio loud AGNs . . . 13

1.5.4 Jet mode and radiative mode AGNs . . . 14

1.6 A Unification scheme for AGNs . . . 15

1.6.1 An AGN’s dissection in the Unification scheme . . . 15

1.6.2 Challenges facing the Unification scheme . . . 18

1.7 Radio interferometery: a tool to observe lightyear-scale regions close to the central engine of AGNs . . . 18

1.7.1 The interferometers we used in this work . . . 20

1.8 Contribution of this thesis. . . 22

2 Radio continuum of galaxies with H₂O megamaser disks: 33 GHz VLA data 25 2.1 Introduction . . . 27

2.2 Sample . . . 28

2.3 Data and data reduction . . . 31

2.3.1 33 GHz observations and data reduction . . . 31

2.3.2 Complementary data . . . 32

2.4 Results. . . 32

2.4.1 Sample . . . 32

2.4.2 Individual sources . . . 34

2.5 Analysis and discussion. . . 38

2.5.1 33 GHz morphologies - where are the jets? . . . 38

2.5.2 Spectral indices . . . 38

2.5.3 Radio continuum versus H₂O disk-maser host galaxy properties . . . 39

2.5.4 Radio continuum, X-ray, and IR luminosities versus maser-disk properties 44 2.6 Summary . . . 50

3 Accretion disk versus jet orientation in H₂O megamaser galaxies 57 3.1 Introduction . . . 58

3.2 Sample . . . 59

3.3 Observations and data reduction . . . 59

3.4 Results. . . 61

3.4.1 Continuum images and previous radio observations . . . 61

3.4.2 A note on the non-detections . . . 66

3.5 Discussion. . . 67

3.5.1 Orientation of the jets with respect to the maser disks . . . 67

3.5.2 Multiscale position angle . . . 69

3.5.3 Radio continuum positions relative to the maser positions. . . 70

3.5.4 Spectral indices and brightness temperatures . . . 72

3.5.5 Radio luminosity versus other properties of the galaxies . . . 73

3.6 Summary . . . 78

3.7 Appendix: Probability distribution of the observed PA offsets . . . 80

4 The nature of radio emission in H₂O -megamaser galaxies 83 4.1 Introduction . . . 83

4.2 Sample . . . 84

4.3 Observations and data reduction . . . 85

4.4 Results. . . 85

4.5 Discussion. . . 86

4.5.1 Spectral indices and brightness temperatures . . . 86

4.5.2 The nature of the radio emission in H₂O -megamaser galaxies . . . . 88

4.5.3 The alignment of the jet with respect to the rotation axis of the maser disk 99 4.6 Conclusion and summary . . . 104

5 Summary of the thesis and outlook 107

vi

Abstract

Galaxies with H₂O -megamaser-disks are low luminosity active galaxies where 22 GHz H₂O - maser emission is detected in their accretion disk surrounding the central suppermassive black hole (SMBH). Furthermore, given that the geometry of the maser disk is known, they provide a unique view of the central region of active galaxies, allowing us to investigate the spatial relationship of the accretion disk with jets on the same physical scales. In this work, we attempt to study the alignment between the radio jet and the associated rotation axis of the sub-pc accretion disks which are traced by the 22 GHz H₂O -megamaser emission. This is an essential part of the paradigm describing active galactic nuclei. Our observations were carried out using radio interferometer arrays with three different resolutions, corresponding to physical scales of∼ 145 pc,∼ 34 pc, and∼ 2 pc at a fiducial distance of 85 Mpc (mean of distances in our sample). Our observations provide a larger sample where morphology or geometry of both accretion disk and jets are observed on similar physical scales.

On kiloparsec scales, our observations were carried out using the Karl G. Jansky Very Large Array (VLA) at 33 GHz. We detect radio emission in 87% of the sources in our sample. We find evidence for biconical extended jets on scales>300 pc for four sources. Seventeen other sources show only one component, often accompanied by extended emission. Among the sources with biconical structure where the maser disk orientation is known, we find that the jet-like 33 GHz continuum feature in one source (NGC 4388) appears to be perpendicular to the maser disk’s orientation.

On scales of∼100 pc, we used the enhanced Multi-Element Radio Linked Interferometer Network (eMERLIN) at 5 GHz to investigate the radio emission. We detected radio emission in 56% of the sources in our sample. Four sources show biconical morphologies, three sources remain unresolved, and three other sources show extended emission. For four of the detected sources, the orientation of the maser disk is known. In all four, the radio continuum is misaligned with the rotation axis of the disk, but by not more than 37^◦relative to the disk’s rotation axis.

On parsec-scales, using the Very Long Baseline Array (VLBA) at 5 GHz, we detect 28%

of the sources, where two sources show only one component, and three other sources show multiple components in the radio maps. Among the five detections, four of them exhibit a maser disk with known orientation. For all four of these sources, the radio continuum is misaligned relative to the rotation axis of the maser disk, but with a 99.1% confidence level, the orientations are not random and are confined to a cone within 32^◦of the maser disk’s normal. Among the four sources the misalignment of the radio continuum with respect to the normal vector to the maser disk is smaller when the inner radius of the maser disk is larger.

Furthermore, based on the spectral indices, brightness temperatures and multi-scale mor- phologies, we conclude that the radio emission in the majority of our sources is dominated by both outflow and star formation. Further observations, such as full polarization studies and kinematic studies of outflows, will help construct a more complete picture of the central region of active galaxies.

Contents

Prologue: Our insignificant presence in the Universe

According to the standard model of cosmology, the Universe is about 13.8 billion years old, and we Homo sapiens on the other hand, are only about 300,000 years old. If we scale the 13.8 billion years old Universe to one year, a unit more understandable for us humans, assuming the big bang happened on January 1^st, the planet Earth was formed in mid-September (Sagan 1977). The first forms of multicellular life appeared on Earth only in mid-December. The first evidence for the appearance of humans indicates that they came around 10:30 pm in the evening of the New Year’s Eve in this one year old Universe. All human history, from domestication of fire, use of stone tools, cave paintings, invention of agriculture to the invention of computers, landing on the moon, searching for extraterrestrial intelligent life, and even the beginning for a possible existential crisis of humans (climate change) occurred only in the last one and a half hours of New Year’s Eve (Sagan 1977). And only in the last minute of this one year old Universe (i.e., 40 seconds in the 1.5 hours existence of humans), we have known that the Universe extends beyond our Galaxy¹.

In the history of human beings, the sky, and especially the night sky, has always aroused curiosity. Until about a 100 years ago, the nature of the fuzzy, cloudy like objects in the sky called nebulae (mostly observable with telescopes), was still uncertain. In 1920, a famous debate on the size of the Universe and the location of the nebulae took place between two astronomers; Harlow Shapley and Heber Curtis. Shapley believed that these nebulae are in the host galaxy of our planet, i.e., the Milky Way, while Curtis argued that they are outside the Milky Way. A few years later, Hubble found the distance to one of these unknown nebulae (M31) using a type of variable star called Cepheids, and proved that extragalactic sources exist (Carroll & Ostlie 2006).

Nowadays we know that there are many billions of other galaxies in the Universe. Figure 0.1shows only a fraction of one degree in the sky², and every fuzzy light source seen in this image is a galaxy (the image contains about 5,500 galaxies). This demonstrates how vast and enormous our Universe is, and we are able to comprehend it, thanks to the developments of our brains’ neocortex, and countless other evolutionary steps. We humans are self-conscious and each of us has the ability to create new thoughts. Nevertheless, our greatest advances have been accomplished through the cooperation with each other, and thanks to our cumulative knowledge, we have learned a great deal about our surroundings, yet still not enough. Among all the species on Earth, humans have the most amount of information stored in their genes and brain (Sagan 1977). As we managed to store information via writing and recording, we are able to progress faster towards understanding ourselves, our surroundings, and the Universe. On this path, we also have encountered countless new questions and mysteries. This thesis wishes to make a very small contribution to our understanding of the Universe and hopefully has lit another spark to shed more light, adding to those already shining.

1Here I attempted to roughly demonstrate how different important events in the history of the Universe are placed in time with respect to each other. The estimated time of the events are not accurate, hence these events could happen a little sooner or later in this re-scaled calendar.

2One degree is roughly the width of your little finger when you stretch your arm in front of you, which is about two times size of the full moon in the sky.

2

Contents

When You and I behind the Veil are past, Oh but the long long while the world shall last,

Which of our Coming and Departure heeds, As much as Ocean of a pebble-cast.

(Omar Khayyam, 11^th-12^thcentury)

Figure 0.1: Hubble eXtreme Deep Field (XDF) image. This image has been made by combining images made with the NASA Hubble Space Telescope over more than 10 years. The combined exposure time is 2 million seconds. Every fuzzy dot seen in this image is a galaxy.©NASA; ESA; G. Illingworth, UCO/Lick Observatory and the University of California, Santa Cruz; R. Bouwens, UCO/Lick Observatory and Leiden University; and the HUDF09 Team.

Structure of this thesis

In Chapter1, I present an introduction to what motivates this thesis. In Chapter2, I present our pilot project aimed to measure the radio continuum emission of H₂O -megamaser galaxies at the kpc-scale. Chapter3focuses on the contribution of this work to investigating the standard paradigm of active galactic nuclei. I discuss the possible dominant sources of the observed radio emission in our sample in Chapter4. Finally, I present the summary and give an outlook in Chapter5.

C H A P T E R 1

Introduction

1.1 What are H

O -megamaser galaxies?

Galaxies have different morphologies; some are elliptical, some are disk-shaped, some lenticular or even irregular. It is generally accepted that in the center of each "normal" galaxy¹exists a very massive object, called a supermassive black hole (SMBH,Kormendy & Richstone 1995).

These SMBHs can have masses between 10⁷to 10¹⁰M². In some galaxies the central SMBH is quiescent, e.g., in our Milky Way, but in other galaxies the SMBH is actively accreting matter.

This matter accretion can be very violent and aggressive which makes the central region of the galaxy shine very bright, even outshining the rest of the galaxy. The central regions of galaxies with an accreting SMBH are called active galactic nuclei or AGNs. Since the AGN’s black holes are surrounded by orbiting gas, if the gas is able to eventually cool, an accretion disk is formed in the equatorial plane (Blandford et al. 2018). Accretion onto the accretion disk is often accompanied by ejection of material to the outside of the disk, via the so called jets. These jets can be empowered and collimated by the central SMBH and the accretion disk, and can expand to intergalactic space at a distance multiple times the size of the galaxy itself. The jets are often invisible in the optical regime, but appear very bright in the radio, and can sometimes be seen in the infrared (IR) and X-rays (see Fig.1.1). Some active galaxies are hosts to a very interesting physical phenomenon that cannot happen naturally on Earth:masers. The microwave amplification by the stimulated emission of radiation (maser) has a similar mechanism as the laser but the radiation is in the microwave regime. Devices yielding maser emission were first built in laboratories (on Earth) in 1953 (Gordon et al. 1954), since this phenomenon cannot occur naturally in environments where thermal equilibrium holds. Under thermal equilibrium, the energy levels of atoms and molecules are populated following a Boltzmann distribution with positive excitation temperature, with N/g=e^−hν/kBTex, where N is the column density, g is the statistical weight of the given state, hνis the energy above the ground state, k_Bis the Boltzmann constant and T_exis the positive excitation temperature. In this case the radiation attenuates while traveling through a medium because it gets absorbed. However, in the interstellar medium the conditions are usually out of thermal equilibrium, and the densities are below the critical density for collisional de-excitation, therefore atoms and molecules can stay in the excited states. An incident photon with energy equal to the energy difference between two levels, can cause decay of one excited atom or molecule and the resulting photon has the same frequency

1In this work, the term galaxy does not refer to dwarf galaxies, which have stellar masses at least two orders of magnitudes less than our Milky Way.

2M, denotes the solar mass which is∼1.98×10³³grams.

Chapter 1 Introduction

Figure 1.1: Hercules A. This elliptical galaxy at the distance of∼two billion light years (686 Mpc, NASA Extragalactic Database) is a strong radio galaxy and an AGN. In this image, the galaxy is seen as a fuzzy yellow ellipse, observed by the Hubble Space Telescope’s Wide Field Camera 3. The radio jets (invisible at optical wavelengths) are shown in red, observed using the Karl G. Jansky Very Large Array (VLA) radio telescope in New Mexico. The jet size is about 1.5 million light-years wide, extending to several times the size of the galaxy.© NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W.

Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA).

and direction as the incident photon. This process is called stimulated emission, which results in the amplification of the radiation. Thus, maser phenomena can naturally occur in space, given the proper physical conditions such as low particle densities are met. Astrophysical masers were first discovered in the Milky Way (Weaver et al. 1965) and later on, also in extragalactic sources such as the galaxy M33 (Churchwell et al. 1977). The line emission from extragalactic masers can be about 10⁶times more luminous than typical masers found in the Milky Way. In this case they are called megamasers. Maser emission is observed from rotational or vibrational transitions in molecules such as OH, H₂O, CH₃OH, HCN, and SiO. In 1993, H₂O megamaser emission, emitted from the water vaporJ_K

−K₊=6₁₆-5₂₃rotational transition at the rest frequency of 22.23508 GHz (λ1.3 cm), was detected with a relatively broadband spectrometer (Nakai et al. 1993). Its spectrum was quite different from others: there were three different groups of lines, one group had a recessional velocity³corresponding to that of the galaxy, and the other groups were blueshifted and redshifted by hundreds of km s⁻¹ with respect to the first group. This was the first discovery of a so-called H₂O -disk-megamaser galaxy (Nakai et al.

1993). Milliarcsecond resolution Very Long Baseline Interferometry (VLBI) showed that the masers are distributed in a sub-parsec⁴sized Keplerian disk surrounding the central engine

3Recessional velocity is the velocity at which an astronomical object is being carried along with its surrounding space due to the expansion of the Universe (see e.g.,Carroll & Ostlie 2006).

4One parsec is about 3.26 light years or 3.08×10¹⁶meters.

6

1.2 The importance of H₂O -megamaser galaxies

Figure 1.2: NGC 4258, the prototype H₂O -megamaser galaxy. The left image is a combination of X-rays (blue), optical (gold), Infrared (red) and radio (purple) observations.©X-ray: NASA/CXC/Univ.

of Maryland/A.S. Wilson et al.; Optical: Palomar Observatory. DSS; IR: NASA/JPL-Caltech; VLA:

NRAO/AUI/NSF. The right hand panel shows the H₂O -megamaser-disk (a warped disk) and the sub-pc radio jet perpendicular to it (Herrnstein et al. 1999).

of the galaxy (see Fig.1.2). Soon after their discovery, H₂O -megamaser galaxies became very important for cosmological studies, since their distances can be measured directly and independently of any cosmological models. In the next section, two important aspects of these fascinating galaxies are described in further detail.

1.2 The importance of H

O -megamaser galaxies

1.2.1 Measuring a fundamental cosmological parameter: the Hubble constant

Our Universe, which contains billions of galaxies, is expanding. The so called Hubble-Lemaître law which formulates this expansion, implies that the recessional velocity (v) of the galaxies is related to their distance (D), and the proportionality constant is called the Hubble constant (H₀):

v=H₀D (1.1)

The inverse of the Hubble constant is a measure of the age of the Universe (Liddle 2015).

Therefore, a precise measurement of H₀is very important for our understanding of the Universe, particularly its age. Over the past 90 years astronomers have determined H₀using different methods. However, the development of new techniques and instruments have recently led to more precise and accurate measurements which resulted in a potential controversy: H₀ measurements from different methods are currently inconsistent within uncertainties. These current measurements indicate a 3.5σdiscrepancy between the measured values from the local Universe and the more distant early Universe (Riess et al. 2016). If this existing inconsistency turns out to be real and not due to systematic issues, its implications could be of great importance to our understanding of the Universe. It could imply that either our understanding of the physics

Chapter 1 Introduction

2000 2002 2004 2006 2008 2010 2012 2014 2016 2018

Publication Year

40 50 60 70 80 90 100

H

[k m s

M pc

]

WMAP1 WMAP3

WMAP5

WMAP7

WMAP9

SH0ES1 SH0ES2 CHP SH0ES3

P13 P15 P18

MCP (mean) KP

MCP

MCP

MCP

MCP

Figure 1.3: H₀measurements since the year 2000. The measurements from the local Universe are shown with teal triangles, from the early Universe (assuming a flat geometry) with orange circles and from H₂O -megamaser-disks with black stars. References for the data:Freedman et al.(2001);Spergel et al.

(2003,2007);Riess et al.(2009);Dunkley et al.(2009);Komatsu et al.(2011);Riess et al.(2011);

Freedman et al.(2012);Bennett et al.(2013);Reid et al.(2013);Kuo et al.(2013);Planck Collaboration et al.(2014);Kuo et al.(2015);Gao et al.(2016);Riess et al.(2016);Planck Collaboration et al.(2016, 2018);Braatz et al.(2018). Plot©FKamali

of the early Universe or our understanding of the local Universe is poor. The latter could be equivalent to having systematic problems in the local distance measurements. Therefore it is important to have as many independent H₀measurements as possible.

Figure1.3shows different measurements of the Hubble constant since the year 2000. In order to measure H₀, one has to measure both the recessional velocity and the distance to the galaxies.

Measuring the recessional velocity is usually not a complicated issue, however, measuring the distance could sometimes be a great challenge. This is where galaxies with H₂O -megamaser- disks come into the picture. These galaxies provide an opportunity to measure their distances directly, and more importantly, independent of cosmological models. Finding more galaxies with H₂O -megamaser-disks is crucial to getting a larger number of H₂O -maser based distance measurements in order to have lower statistical uncertainties. The Megamaser Cosmology Project (MCP) was initiated to measure H₀within 3% accuracy using at least 9 galaxies hosting H₂O -megamaser-disks. The H₂O -megamasers are mostly associated with two types of active galaxies called Seyfert 2 and LINERs (low-ionization nuclear emission-line regions), thus the MCP has surveyed more than 4000 such galaxies to find suitable H₂O -megamaser-disks in order to determine their geometric distances. The survey resulted in a.1% detection rate, only

∼9 are potentially good candidates that are located well into the Hubble flow where the motion of galaxies due to the expansion of the Universe dominates other motions (see e.g.,Kuo et al.

2018). Since the megamaser emission is very bright, using radio interferometry techniques (see Sect.1.7) the physical size of the Keplerian disks could be measured. The distance to the host galaxies could also be measured directly using the angular diameter distance D, which relates

8

1.2 The importance of H₂O -megamaser galaxies

2θ V

θ

2V

Figure 1.4: The H₂O -megamaser-disk in NGC 4258. The angular diameter distance (D) is measured using the velocity corresponding to radius r (V_r), the acceleration (a=^V_r^2r), and the disk’s angular size (θ, measured using the rotation curve as shown here).©JBraatz, reconstructed by FKamali based on Miyoshi et al.(1995);Greenhill et al.(1995);Herrnstein et al.(1998).

the actual size of an object (here the physical size of the maser disk, r) to its apparent angular size,θ:

D= r

θ (1.2)

In practice, for the simplest model, the distance is determined by three parameters:

• θ, the disk’s angular size which is measured from the rotation curve as shown in Fig.1.4,

• V_r, the velocity corresponding to the radius r,

• a, the acceleration of the systemic features measured by the velocity drift of the systemic masers (a= ^V_r^2r). This can be determined by monitoring the spectrum.

The MCP has so far published distance measurements to four galaxies (Reid et al. 2013;Kuo et al. 2013,2015;Gao et al. 2016), which resulted in a weighted average value of H₀=69.3±4.2 kms⁻¹Mpc⁻¹(Braatz et al. 2018).

In addition to their cosmological importance, studying H₂O -megamaser-disks plays a crucial role in testing the AGN’s standard paradigm, explained in the following section.

1.2.2 Investigating jet-disk connection

A unique feature of galaxies hosting H₂O -megamaser-disks is that their sub-parsec accretion disk can be studied using the H₂O -maser emission. As mentioned before, the accretion of

Chapter 1 Introduction

material onto the SMBH is often accompanied by the ejection of material (jets) along the rotation axis of the disk. Studies in the late 90s, have shown that the jets are not necessarily perpendicular to the large scale galactic disk in active galaxies (Pringle et al. 1999;Nagar &

Wilson 1999;Kinney et al. 2000;Schmitt et al. 2001). A warp in the accretion disk could cause this, or the gas fueling the SMBH has a different angular momentum direction than that of the gas on a galaxy wide scale. Other studies investigated the position angles (PAs) on different scales and reported that the different PAs do not necessarily align (Schmitt et al. 2002;Greene et al. 2013;Pjanka et al. 2017). In other words, the direction of angular momenta for disks on different scales do not align. When determining the jet directions, we are biased by the viewing angle and corrections are needed. The jets might be moving towards or away from our line of sight and may thus also be affected by Doppler boosting if they reach relativistic speeds. However, if the accretion disk is viewed edge-on, the putative jets should be in the plane of the sky and free of viewing angle biases. In H₂O -megamaser-disk galaxies exhibiting the above mentioned groups of systemic, red- and blueshifted features, the maser disk is viewed edge-on, since the coherent amplification of radiation requires large linear scales. Therefore, we only observe the maser emission if the amplification path is directed towards our line of site.

Although H₂O -megamasers are observed mostly in Seyfert 2s, where a dusty obscuring torus surrounds the accretion disk around the AGNs and blocks the view in the optical or near IR regime, the radio waves from the masers can penetrate the obscuring torus and are not affected by extinction. Therefore, H₂O -megamaser galaxies are good laboratories to study the AGN’s paradigm, where the central engine accretes matter and the accretion is associated with ejection of material into the space, the so called jets which should be perpendicular to the plane of the disk.

As mentioned above, the host galaxies of H₂O -megamaser-disks are Seyfert 2s and LINERs.

In the following sections, I briefly review what an AGN is.

1.3 Active galactic nuclei, a brief history

AGNs were observed for the first time in 1908, by Edward A. Fath. He was observing the spectra of the brightest “spiral nebulae” when he discovered some objects showing broad emission lines that at that time were known to be characteristics of gaseous nebulae. It should be noted that in 1908, it was not yet known that extragalactic objects exist. Herbert Curtis at the Lick Observatory, describes one other nebula (called M 87) in 1918 and mentions not only a lack of spiral arms, but also a straight line of emission connected with the core of the galaxy (Curtis 1918). This was the first reported jet, also at a time when the extragalactic origin of many nebulae were not yet known. Years later, when astronomers knew that the Universe extends beyond our Galaxy and that there are billions of other galaxies, Carl K. Seyfert published a paper (Seyfert 1943) which showed that a fraction of galaxies exhibit broad atomic emission lines which cover quite different degrees of ionization and originate from a small nucleus which look like a star (i.e., it looked point-like on a photographic plate). However these galaxies did not attract attention until the late 1950s, when a source catalog at radio wavelengths led to the discovery of sources which looked like stars on photographic plates but had broad emission lines at unknown wavelengths in their spectra which could not be from a star (the spectra from stars in thermal emission follow Planck’s law). Hence these sources were called quasi stellar radio sources (quasars). Later on, the lines were identified as highly redshifted hydrogen emission lines. These high redshift sources also had high luminosity, and the important question was what is the nature of these sources? The most acceptable explanation was reported bySalpeter (1964) andZel’dovich(1964): the accretion of matter onto a central SMBH is responsible for

10

1.4 What defines an active galaxy?

the high luminosity of quasars. During the last 70 years, countless number of studies were carried out, with the purpose of understanding the nature of AGNs. Here, we review a few important aspects of these various studies, such as AGN classifications and the unified model of AGNs which try to describe the different observed properties (and consequently the different classes) using only a few fundamental parameters.

1.4 What defines an active galaxy?

AGN hosts are defined as galaxies which contain a massive (>10⁵M) accreting black hole (BH) with Eddington ratios greater than 10⁻⁵(Netzer 2015). The Eddington ratio is the ratio of the bolometric luminosity of the galaxy and the Eddington luminosity, where Eddington luminosity is defined as the luminosity at which the gravitational and radiation forces are in balance. Some observational properties are common within the whole family of AGNs. For example, their spectral energy distribution (SED) extends from radio up toγ-rays (i.e.∼10¹²Å to∼10⁻⁴Å, Elvis et al. 1994, see Fig.1.5). In comparison, the spectrum of normal galaxies devoid of an AGN is confined to a range from∼2×10⁴Å to∼4×10³Å, since their light is the superposition of stellar and interstellar dust lights. AGN spectra are dominated by emission lines, usually high excitation lines with broad line widths. These emission lines, as well as the continuum emission from AGNs, are highly variable on scales of a few hours to many years, depending on the wavelength of study. This variability is associated to a small nuclear region. AGNs are sometimes strong radio sources (∼10% of them, see Sect.1.5.3) but the best frequency range to search for AGNs is in the X-rays. They are luminous in the X-rays, with a total luminosity of about 10¹²L⁵(Osterbrock & Ferland 2006). While this is a general definition of an AGN, there are various classes and sub-classes which possess different properties but they all have in common the above mentioned features. In the following section, I will explain some relevant classification and unification schemes of AGNs, referring to relevant references for further detail.

1.5 Active galactic nuclei classification

It is a difficult task to demonstrate a well-established classification of AGNs. There exists a variety of different classifications based on radio or optical luminosities, or the strength of emission lines. Unification schemes try to combine all these different classes into a single one using only a few parameters such as viewing angle, luminosity and Eddington ratio. In the following, I review three main AGN classifications schemes: one based on the spectral properties, another one based on the ratio of radio to optical luminosity, and a third one based on the accretion mode of the central engine.

1.5.1 Type 1 and Type 2 AGNs

One feature plays an important role in the classification of AGNs: the width of the observed emission lines. If the permitted emission lines⁶have widths&1000 km s⁻¹, and the forbidden emission lines⁷with widths<1000 km s⁻¹are observed in the spectrum, the galaxies are called

5Ldenotes solar luminosity, equivalent to 3.828×10²⁶W or 3.828×10³³erg s⁻¹.

6Permitted lines are transitions of electrons between different levels that are allowed by quantum-mechanical selection rules.

7Forbidden lines are those transitions that are not allowed by the electric-dipole quantum-mechanical selection rules, but could occur for example by magnetic-dipole transitions (see e.g.,Osterbrock & Ferland 2006). These

Chapter 1 Introduction

Table 1.1: Spectral properties of different classes of AGNs. Table taken fromKamali(2014), based on the lecture notes from the Active Galactic Nuclei course held in the University of Göttingen in winter-semester 2012.

Class Sub-class Host Galaxy

Description

Seyfert Type 1 spirals strong broad permitted lines, narrow forbidden lines, Fe II lines with width comparable to Hβ, hard X-ray power law with a soft excess

Type 1.5 spirals strong broad and narrow lines with comparable strength, weak Fe II lines

Type 1.8 spirals weak broad lines

Type 1.9 spirals weak Hα, no higher order Balmer lines

Type 2 spirals only narrow lines, no Fe II lines, no soft X-ray but hard X-rays NLS1⁸ spirals almost like Sy1 but with narrower Balmer lines (permitted lines

are slightly broader than the forbidden ones), stronger Fe II and weaker [OIII] lines, lines are narrow due to the lower black hole mass compared to that of broad line Sy1s

Quasars Radio-loud all strong radio emission, some polarization, broad and narrow emis- sion lines

Radio-quiet all weak radio emission, weak polarization, broad and narrow emission lines

Radio Galaxies

lobe domin- ated

FR I⁹: lower luminosity relative to FR II, brighter in the lobe than the cores, decreasing surface brightness toward the edge

FR II: more luminous, powerful lobes, jet less obvious,L_lobs ∼ L_core

SSRLQs¹⁰ Core domin-

ated: Blazars

90% in el- lipticals

BL Lacs¹¹: almost absence of emission lines, featureless optical spectrum, rapid variability at all energies even on short scales OVV quasars¹²: broad optical emission lines, rapid variability FSRLQs¹³: broad emission lines, radio spectral slopeαr = 0, higher redshift than BL Lacs

LINERs¹⁴ all similar to Sy2 but with low luminosity and low ionization emission lines like [OI], [OII], [NII], [SII]

type 1 AGNs. On the other hand, there are AGNs which lack the broad permitted emission lines in their spectra, and are called type 2 AGNs. Seyfert galaxies which are one of the first classes of AGNs discovered, are also divided into two main sub-classes, i.e., of type 1 and type 2, based on the same criteria (absence or presence of broad emission lines in their spectra). For Seyfert

lines are produced with much reduced intensities compared to permitted lines, and in environments with low densities, these transitions may become observable.

8Narrow Line Seyfert 1 (NLS1), seeOsterbrock & Pogge(1985)

9Fanaroffand Riley (FR), seeFanaroff& Riley(1974)

10Steep-Spectrum Radio Loud Quasars (SSRLQs)

11BL Lacertae

12Optically Violently Variable (OVV) quasars

13Flat-Spectrum Radio Loud Quasars (FSRLQs)

14Low Ionization Emission-line Regions (LINERs), seeHeckman(1980)

12

1.5 Active galactic nuclei classification

Figure 1.5: Spectral energy distribution of AGNs (Elvis et al. 1994). As seen in this spectrum, the AGN’s SED spans from the low energy radio regime to the very high energy X-ray regime.

galaxies, there are also intermediate sub-classes, whose details are presented in Table1.1. In these intermediate sub-classes, the strength of some broad lines such as Fe II or some hydrogen emission lines, e.g., the Balmer series, are different.

1.5.2 Low excitation and high excitation AGNs

Optical spectroscopic properties are also used to classify AGNs into two classes of high excitation and low excitation AGNs. This classification is based on the existence of strong emission lines in the spectra such as [O II]λ3727, [O III]λ5007 and [Ne II]λ3867, or the existence of weak emission lines such as [O II]λ3727 (Hine & Longair 1979). In general, galaxies with high excitation emission lines are called HEG, and galaxies with low excitation emission lines in their optical spectra are called LEG. Quasars and Seyfert galaxies are HEGs while LINERs belong to the LEG subclass.

1.5.3 Radio quiet and radio loud AGNs

With respect to their properties at long wavelengths, AGNs are also grouped into two classes, those that are radio loud and those that are radio quiet. As the name suggests, the criteria that define to which group an AGN belongs, is the luminosity of the galaxy in the radio regime. The radio loud galaxies produce large scale radio jets and lobes, and the jet contributes a significant fraction to the total bolometric luminosity of the galaxy. The radio loud galaxies make up about 10% of the AGNs and are usually found in ellipticals, while the radio-quiet AGNs prefer spirals (Wilson & Colbert 1995).

Chapter 1 Introduction

Figure 1.6: Classification of AGNs, taken fromTadhunter(2008).

Traditionally, the radio loudness is defined as the ratio of the monochromatic radio luminosity to the monochromatic optical luminosity (R). For instance, R can be defined as:

R= f (5 GHz)

f (4400 Å) (1.3)

For radio loud galaxies this ratio is greater than 10 (Kellermann et al. 1989). Fig.1.6shows the sub-classes of radio loud and radio quiet AGNs. Seyfert 2s and LINERs which are host to H₂O -megamaer-disks, are among the radio quiet AGNs.Padovani(2016) has argued that the two classes of radio quiet and radio loud AGNs are intrinsically different objects, the major physical difference being lack or presence of relativistic radio jets. Therefore,Padovani(2016) suggested that using the names "radio loud and radio quiet" is misleading, and the subclasses must be called "jetted" and "non-jetted" AGNs. In this work, we stay with the traditional classification of radio quiet/loud AGNs.

1.5.4 Jet mode and radiative mode AGNs

Heckman & Best(2014) suggested that the AGNs could be classified in two groups, based on the dominant energy output resulting from extracting energy from the potential well of a SMBH: AGNs in a radiative-mode or in a jet-mode. In the radiative-mode AGNs, dominant energetic output is in the form of electromagnetic radiation produced by the gas accreted by the SMBH (i.e., from the accretion disk). Jet-mode AGNs produce relatively little radiation from accretion, and energetic output takes the form of the bulk kinetic energy transported in

14

1.6 A Unification scheme for AGNs

Figure 1.7: AGN classification. Figure taken fromHeckman & Best(2014).

collimated outflows. Fig.1.7 shows the different classifications in an organized and easily readable way.

1.6 A Unification scheme for AGNs

It was suggested in the 1980s, that the two classes of type 1 and type 2 AGNs are the same, but viewed from different angles (Antonucci & Miller 1985;Barthel 1989;Urry & Padovani 1995).

In the unified model of AGNs, the central engine is surrounded by obscuring gas and dust, the so called torus. This material is optically thick and when looking through the torus, the broad emission lines are absorbed. However, they can still be seen in the reflected polarized light (Antonucci & Miller 1985). In the unified model of AGNs, blazars are those AGNs where one part of the two-sided jet is directed towards us.

1.6.1 An AGN’s dissection in the Unification scheme

The unified model of AGNs suggests that an AGN consists of the following parts (see Fig.1.8):

• The SMBHis the central engine of AGNs characterized by its high mass and small Schwarzschild radius. Figure1.9shows the first picture of a dark shadow caused by gravitational light bending and photon capture at the event horizon of the SMBH at the center of galaxy M 87 (Event Horizon Telescope Collaboration et al. 2019).

Chapter 1 Introduction

1 Clouds in Narrow Line Region (NLR) SMBH and the accretion disk

Seyfert 2

Seyfert 1

Geometry Taxonomy

Clouds in Broad Line Region (BLR)

Blazar

Jet

Torus

Figure 1.8: Structure of an AGN, where the accretion disk is viewed edge-on and the jets are in the plain of the sky. The viewing angle to the accretion disk is different in Seyfert 2s, Seyfert 1s and blazars as presented in this figure. Different parts of the AGN are not to scale.©FKamali

• TheAccretion diskis a structure surrounding the SMBH, where the rest mass energy of infalling material is converted into radiation or fast particles (Osterbrock & Ferland 2006). The accretion disk is visible in the optical/ultraviolet (UV) regime, and sometimes also when applying 22 GHz H₂O radio spectroscopy. It is believed that a putativecorona (which is an atmosphere above the inner accretion disk with unknown geometry) accom- panies the accretion disk. The corona is responsible for inverse Compton scattering of the accretion disk photons to X-ray energies (Padovani et al. 2017).

• Torusis an obscuring structure surrounding the SMBH. The radiation from the torus peaks in the mid-infrared (MIR). The more modern view of the torus implies that the dust is clumpy and compact, since a smooth dust distribution cannot survive in the region close to the central engine of AGNs (Almeida & Ricci 2017, and references there in).

MIR interferometery observations constrain the size and geometry of the torus (see Fig.1.10).

• Thebroad Line Region (BLR)is in the gravitational potential of the SMBH. The size of this region can vary from a few light hours to a few light days (Kaspi et al. 2005). The electron number densities are& 10⁹cm⁻³in this region (Osterbrock & Ferland 2006), therefore no forbidden lines are produced here. Forbidden lines are low probability transitions from atoms which only become relevant in a low density medium.

• TheNarrow Line Region (NLR)is outside the torus and not in the gravitational potential of the SMBH. Therefore the width of emission lines is smaller, typically ranging from 200 km s⁻¹to 500 km s⁻¹. Both permitted and forbidden lines can form here, due to lower

16

1.6 A Unification scheme for AGNs

Figure 1.9: The first image of a black hole, located at the center of the galaxy M 87. This image was constructed using the Event Horizon Telescope.©Event Horizon Telescope Collaboration.

Figure 1.10: AGN’s structure seen along the equatorial and polar direction. Different colors indicate different densities or compositions. Figure taken fromAlmeida & Ricci(2017).

electron densities of order∼10⁴cm⁻³(Osterbrock & Ferland 2006). The NLR extends to kiloparsec-scales.

• The radiojets and lobesare observed in radio loud (or "jetted") AGNs.

Chapter 1 Introduction

1.6.2 Challenges facing the Unification scheme

The unification scheme presented in the late 1980s is currently facing some challenges: true type 2 AGNs are galaxies with broad emission lines not detected in the polarized light. These galaxies represent about 30% of type 2 AGNs in the local Universe, which is considerably a large number (Netzer 2015, and references therein.). The lack of observed broad emission lines in polarized light of these galaxies could indicate that some type 2 AGNs are intrinsically different (i.e., the BLR does not exist). Furthermore, the orientation alone cannot explain the different covering factors measured for type 1 and type 2 AGNs (e.g.,Mateos et al. 2016). This implies that the nature of the obscuring tori in type 1 and type 2 AGNs is different.

The radio regime is one of the most salient frequencies to observe the AGNs, because of the high angular resolution provided by interferometric radio observations. More specifically, observing the central region of galaxies at distances>50 Mpc requires high resolutions which cannot be achieved in the optical, IR or X-rays. However, using radio interferometry we can resolve the pc-scale regions of AGNs without being hindered by obscuration from dust particles, which emit photons at substantially shorter wavelengths than the emission we are studying here at radio wavelengths.

1.7 Radio interferometery: a tool to observe lightyear-scale regions close to the central engine of AGNs

The text in this section is largly based on Parsons(2013),Klein(2014),Condon&Ransom (2016), andWilson et al.(2009).

The angular resolution of instruments has always been a subject of constant improvement in astronomy. The angular resolution (θ) of a telescope is determined by the wavelength of the observation (λ) and the diameter (D) of the telescope:θ ∝ λ/D. When observing in the radio regime, the wavelengths are longer (mm to cm) and to obtain high resolution (i.e., a lower value of θ), one needs to have telescopes with very large diameters. For instance, at 20 cm, with a 100 m single dish telescope, e.g., Effelsberg, the highest resolution that can be achieved is on the order of∼arcminutes. In order to obtain the resolution of 1 arcsecond, we need a diameter of∼ 40 km. However, building fully steerable single dishes with diameters

& 100 m is not achievable with the current technologies. With radio interferometery and

aperture synthesis techniques, the distance between two telescopes, called baseline (B), replaces the telescopes diameters and therefore achieving higher resolutions is affordable (θ ∝ λ/B).

Current interferometers even include baselines as large as the radius of a satellite’s orbit moving around the Earth (e.g., the Radio Astron satellite). Here we briefly review the principles of radio interferometry and aperture synthesis. We start with a simple interferometer of two antennas, 1 and 2, separated by a distance of B (see Fig.1.11). Since the antennas are separated, they will not receive the signals from the sky at the same time. The antenna closer to the source will receive the signal first. The other antenna will receive the signal with a so called geometric delay,τ_g. This delay is equal to ^B.s_c , whereBis the base line,sis the unit vector pointing toward the source andcis the speed of light (we present vectors in boldface). For different sources in the sky, the unit vectorsis different, therefore the geometrical delay will change. Since the astronomical sources are far away, the signals received by the telescopes are plane waves. A signal therefore reaches antenna 1 at timet−τ_g=t− ^B.s_c , and antenna 2 at time t:

V₁=A cos(ω(t−τ_g)) (1.4)

18

1.7 Radio interferometery: a tool to observe lightyear-scale regions close to the central engine of AGNs

s

B τ_g=B.s

c B.s

Antenna1 Antenna2

Figure 1.11: A two-element interferometer, consisting of two identical antennas, 1 and 2. The unit vectorsis pointing towards the source, and the vectorBis the baseline. Antenna 2 will receive the signal with a delay ofτ_g=^B.s_c .©FKamali

V₂= A cos(ωt) (1.5)

whereω=2πν(thereforeωτ_g=2πντ_g=2πB.s/λ), andAis the amplitude. These two signals are correlated, i.e., multiplied and averaged in the so-called correlator, to get the final signal.

V₁V₂=A²cos(ωt)cos(ω(t−τ_g))=(A²

2 ) (cos(2ωt−ωτ_g)+cos(ωτ_g)) (1.6) The term cos(2ωt−ω τ_g) is rapidly varying and can be neglected when averaging over a sufficiently large interval (∆t(2ω)⁻¹). The correlator’s response is:

R=<V₁V₂>=A² 2

cos(ωτ_g)=A² 2

cos 2πB.s

λ

=R_cos (1.7)

This implies that the correlator’s output changes sinusoidally across the sky as the source direction changes due to the Earth’s rotation. The interferometer is sensitive to different scales depending on the length of the baseline, and only sensitive to the directions perpendicular to the baseline. To receive information on the smaller scale structure of the sources, we need longer baselines, while shorter baselines provide information on the large scale structures. However, since cosine is an even function, it cannot recover the odd part of the source function. We need a sinus response as well, to recover the whole source function. This can be achieved by introducing a 90^◦phase shift in one of the signals. After correlating as described above, we obtain:

R_sin=<V₁V₂>=A² 2

sin(ωτ_g) (1.8)

Chapter 1 Introduction

We can combine Eq.1.8and1.7into a complex visibility defined as:

V≡R_cos−i R_sin= Be^−iΦ (1.9)

whereB=(R²_cos+R²_sin)^1/2is the visibility amplitude andΦ =tan⁻¹(R_sin/R_cos) is the visibility phase. Now we would like to know what the response of our interferometer is to an extended source with brightness distribution ofI_ν, which falls into the primary beam of the correlated antennas. In order to understand this, we define new coordinate systems for the baselines on the ground and the source in the sky.

We define^B_λ ≡(u, v, w), i.e., the baseline length in units of wavelength, whereuis in east-west direction,vis in north-south direction, andwis the vertical component in the up-down direction.

We also defines ≡ (l,m, p

1−l²−m²), i.e., the source unit vector is split into an east-west direction on the sky (l), a north-south direction (m) and a third component that can be written in terms oflandmsincesis a unit vector. For each point on the sky, we have a phase term that is defined by the coordinates on the sky (l, m, p

1−l²−m²) and is a function of the baseline (u, v, w). Our visibility is the sum over the two primary beams (of the individual antennas) over the whole sky:

V_(u,v,w)= Z Z

A(l,m)I_ν(l,m)e^{−2πi(ul+vm+w}

√

1−l²−m²)dldm (1.10) Separating thewterm, we will have

V_(u,v,w)=Z Z

A(l,m)I_ν(l,m)e^{−2πi(ul+vm)}.e^−2πiw

√

1−l²−m²

dldm (1.11)

HereAis the response of the primary beams of the antennas as a function oflandm. The product A(l,m)I_ν(l,m) is called the "perceived sky". Let’s assume that our antennas are separated in a two-dimentional plane, thus thewterm is zero and can be ignored here. Then we can see that what we observe is the true sky through the response of our antenna. In other words, the visibility is the Fourier transform of the perceived sky; (l,m) and (u, v) are Fourier pairs. The Fourier transform of the image that we made in (l,m) coordinates is called the uv-plane. This uv-plane is sampled by the baselines in our interferometer array. Finally, the inverse Fourier transform of the sampled uv-plane gives us an image. As the Earth rotates, the position of the antennas are changed with respect to the source over time. Therefore, depending on the elevation of the source, circles and ellipses are sampled in the uv-plane, creating more Fourier components of the brightness distribution. This technique is called aperture synthesis. In conclusion, the more data points in the uv-plane, the better the image quality. The uv-coverage depends on the number of antennas and their distribution, the elevation of the sources, and the observation’s period.

Next I review what interferometers were used in this work, and give examples of their uv-coverage in Fig.1.12. The data reduction procedure of our data is explained in each chapter individually.

1.7.1 The interferometers we used in this work

Karl G. Jansky Very Large Array

The Karl G. Jansky Very Large Array (VLA) is an array consisting of 27 antennas, each one 25 meters in diameter, and deployed in a Y-shaped array (see Fig.1.13). The VLA, which

20

1.7 Radio interferometery: a tool to observe lightyear-scale regions close to the central engine of AGNs

Figure 1.12: UV-coverage for the galaxy Mrk 0001, using three different interferometer arrays we have used in this work:top: VLA,middle: eMERLIN,bottom: VLBA. The number of antennas, their distribution and the integration time of the observations resulted in different uv-coverage.

Chapter 1 Introduction

is located in New Mexico (U.S.), has 4 different configurations, where the maximum baseline varies between 1 km and 36 km.

Figure 1.13: An aerial view of the Karl G. Jansky Very Large Array shows its Y-shape.

©NRAO/AUI/NSF

Very Long Baseline Array

The Very Long Baseline Array is an interferometer consisting of 10 antennas, each 25 meters in diameter, distributed across the U.S. (see Fig.1.14). The telescopes are controlled remotely from the control center in Socorro, New Mexico. The longest baseline provided by the VLBA is∼8000 km, therefore the VLBA observations have very high angular resolution.

Enhanced Multi-Element Radio Linked Interferometer Network

The enhanced Multi-Element Radio Linked Interferometer Network (eMERLIN) is an ar- ray consisting of up to 7 antennas with different diameters spread across the UK, providing a longest baseline of∼ 217 km (see Fig.1.15). The array is operated from the Jodrell Bank Observatory by the University of Manchester. The signals are correlated at Jodrell Bank.

1.8 Contribution of this thesis

In this work, we investigate the standard paradigm of AGNs by studying the radio continuum of H₂O -megamaser galaxies on different scales from sub-parsec to kiloparsec. In addition, we study the alignment of the jets with respect to the rotation axis of the accretion disk, and probe the origin of radio emission in our sample. Furthermore, we search for any correlation between the radio continuum properties of the galaxies and the properties at other frequencies, in order to make future surveys looking for H₂O -megamaser galaxies more efficient. Our sample of 24 H₂O -megamaser galaxies is free of viewing angle biases for the jet, and the maser disk’s position angle is measured accurately, providing us with a perfect sample for investigating the

22

1.8 Contribution of this thesis

Figure 1.14: The Very Long Baseline Array consists of ten radio telescopes, each with a dish 25 meters in diameter spread over the U.S.©NRAO

Figure 1.15: The enhanced Multi-Element Radio Linked Interferometer Network, located in the UK, is an interferometer consisting of up to 7 antennas. Image taken from http://www.merlin.ac.uk/.

jet-accretion disk connection.

C H A P T E R 2

Radio continuum of galaxies with H ₂ O megamaser disks:

33 GHz VLA data

The content of this chapter, with the same title, was published as an article in the Astronomy&Astrophysics journal.

Credit:F. Kamali et al., A&A, 605, A84, 2017, reproduced with permission©ESO.

Abstract

Context.Galaxies with H₂O megamaser disks are active galaxies in whose edge-on accretion disks 22 GHz H₂O maser emission has been detected. Because their geometry is known, they provide a unique view into the properties of active galactic nuclei.

Aims. The goal of this work is to investigate the nuclear environment of galaxies with H₂O maser disks and to relate the maser and host galaxy properties to those of the radio continuum emission of the galaxy.

Methods.The 33 GHz (9 mm) radio continuum properties of 24 galaxies with reported 22 GHz H₂O maser emission from their disks are studied in the context of the multiwavelength view of these sources. The 29–37 GHz Ka-band observations are made with the Jansky Very Large Array in B, CnB, or BnA configurations, achieving a resolution of∼0.2 - 0.5 arcseconds. Hard X-ray data from theSwift/BAT survey and 22µm infrared data from WISE, 22 GHz H₂O maser data and 1.4 GHz data from NVSS and FIRST surveys are also included in the analysis.

Results.Eighty-seven percent (21 out of 24) galaxies in our sample show 33 GHz radio con- tinuum emission at levels of 4.5 – 240σ. Five sources show extended emission (deconvolved source size larger than 2.5 times the major axis of the beam), including one source with two main components and one with three main components. The remaining detected 16 sources (and also some of the above-mentioned targets) exhibit compact cores within the sensitivity limits. Little evidence is found for extended jets (>300 pc) in most sources. Either they do not exist, or our chosen frequency of 33 GHz is too high for a detection of these supposedly steep spectrum features. In NGC 4388, we find an extended jet-like feature that appears to be oriented perpendicular to the H₂O megamaser disk. NGC 2273 is another candidate whose radio con- tinuum source might be elongated perpendicular to the maser disk. Smaller 100–300 pc sized jets might also be present, as is suggested by the beam-deconvolved morphology of our sources.

Whenever possible, central positions with accuracies of 20-280 mas are provided. A correlation analysis shows that the 33 GHz luminosity weakly correlates with the infrared luminosity. The 33 GHz luminosity is anticorrelated with the circular velocity of the galaxy. The black hole

Chapter 2 Radio continuum of galaxies with H₂O megamaser disks:

33 GHz VLA data

masses show stronger correlations with H₂O maser luminosity than with 1.4 GHz, 33 GHz, or hard X-ray luminosities. Furthermore, the inner radii of the disks show stronger correlations with 1.4 GHz, 33 GHz, and hard X-ray luminosities than their outer radii, suggesting that the outer radii may be affected by disk warping, star formation, or peculiar density distributions.

Key words.galaxies: active – galaxies: jets – galaxies: nuclei – radio continuum: galaxies – galaxies: ISM – galaxies: Seyfert

26

2.1 Introduction

2.1 Introduction

In 1969, the 22 GHz (λ∼1.4 cm) H₂O maser line that is emitted from the water vapor 6₁₆-5₂₃ rotational transition was detected for the first time in the Milky Way (Cheung et al. 1969). Since then, this interesting phenomenon has been investigated in a variety of relevant astrophysical environments, also including sources far outside the Milky Way (e.g.,Impellizzeri et al. 2008).

One highlight was the discovery of high-velocity H₂O maser emission in the nucleus of NGC 4258, offset by±1000 km s⁻¹from their host galaxy in systemic velocity (e.g.,Nakai et al.

1993;Miyoshi et al. 1995;Herrnstein et al. 1999). This led to the discovery of so-called disk masers, which trace subparsec edge-on Keplerian disks that surround supermassive black holes (SMBHs). Later on, the Keplerian motion was used to determine the SMBH masses and direct angular diameter distances. The distance measurement to NGC 4258 is not only independent of the traditional distance ladder, but is also among the most accurate in extragalactic space (Herrnstein et al. 1999;Humphreys et al. 2013). The importance of such direct geometrical extragalactic distance measurements provided the motivation to carry out surveys for finding more H₂O maser disks (e.g.,Braatz et al. 2004).

The high accuracy of distances obtained from this method can reduce uncertainties in the Hubble constant. With this idea in mind, the Megamaser Cosmology Project (MCP) was initiated, with the purpose of measuring the Hubble constant with 3% accuracy. In the framework of the MCP, thousands of galaxies have been searched for H₂O megamaser¹ emission. To date, about 160 galaxies with H₂O megamaser emission are known, 39 of which are disk-maser candidates² (32 “clean” disk masers, where “clean” means that the maser emission arises from an edge-on Keplerian disk that dominates other emission from nuclear jets or outflows, seePesce et al. 2015). While the number of disk masers is low, (∼1 % of local Seyfert 2s and low ionization nuclear emission region (LINER) galaxies, see, e.g.,Van den Bosch et al. 2016, and references therein), their unique geometrical properties, such as an edge-on disk with a putative jet in the plane of the sky, provide motivation to investigate the host galaxies of these H₂O megamaser disks in more detail in order to better understand their nuclear environment. Investigating the radio continuum of these galaxies can reveal emission from inside the maser disks as well as jets or outflows in the vicinity of the central black hole. By definition, these galaxies are particularly suited for studying the accretion disk–jet paradigm under extremely well-defined boundary conditions, including the knowledge of distance, inclination of the accretion disk, and mass of the SMBH.

Here, we present radio continuum data observed at a frequency range centered at 33 GHz (9 mm wavelength) from 24 such disk-maser sources, obtained with the Karl Jansky Very Large Array (VLA)³. The purpose of our investigations is to compare the geometry and luminosity of the parsec-scale maser disk with nuclear radio continuum properties, as well as to probe these galaxies for large-scale (kpc scale) radio jets. Resolving structure at kpc scale requires an angular resolution of 0.2-0.5 arcseconds at the distances of our sources. To achieve this resolution, we chose the Ka band of the higher frequency bands, which uses frequencies not too far from the 22 GHz H₂O maser line (to obtain a realistic idea of the radio continuum distribution and intensity near the frequency of the H₂O maser). At the same time, this band minimizes the atmospheric attenuation (which may be stronger at 22 GHz).

1Extragalactic masers are about a million times more luminous than many Galactic masers, hence they are called megamasers.

2See the MCP webpage:https://safe.nrao.edu/wiki/bin/view/Main/MegamaserCosmologyProject

3The Karl Jansky Very Large Array (VLA) is a facility of the National Radio Astronomy Observatory (NRAO), which is operated by the associated universities, Inc., under a cooperative agreement with the National Science Foundation (NSF).